Cache polymorphic properties

src/System.Text.Json/src/System/Text/Json/Serialization/JsonClassInfo.AddProperty.cs

@@ -2,10 +2,11 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using System.Collections.Generic;
+using System.Collections.Concurrent;
 using System.Diagnostics;
 using System.Reflection;
 using System.Text.Json.Serialization;
+using System.Threading;
 
 namespace System.Text.Json
 {
@@ -209,18 +210,27 @@ internal JsonPropertyInfo CreateRootObject(JsonSerializerOptions options)
                 options: options);
         }
 
-        internal JsonPropertyInfo CreatePolymorphicProperty(JsonPropertyInfo property, Type runtimePropertyType, JsonSerializerOptions options)
+        internal JsonPropertyInfo GetOrAddPolymorphicProperty(JsonPropertyInfo property, Type runtimePropertyType, JsonSerializerOptions options)
         {
-            JsonPropertyInfo runtimeProperty = CreateProperty(
-                property.DeclaredPropertyType, runtimePropertyType,
-                property.ImplementedPropertyType,
-                property.PropertyInfo,
-                parentClassType: Type,
-                converter: null,
-                options: options);
-            property.CopyRuntimeSettingsTo(runtimeProperty);
+            static JsonPropertyInfo CreateRuntimeProperty((JsonPropertyInfo property, Type runtimePropertyType) key, (JsonSerializerOptions options, Type classType) arg)
+            {
+                JsonPropertyInfo runtimeProperty = CreateProperty(
+                    key.property.DeclaredPropertyType, key.runtimePropertyType,
+                    key.property.ImplementedPropertyType,
+                    key.property.PropertyInfo,
+                    parentClassType: arg.classType,
+                    converter: null,
+                    options: arg.options);
+
+                key.property.CopyRuntimeSettingsTo(runtimeProperty);
+
+                return runtimeProperty;
+            }
+
+            ConcurrentDictionary<(JsonPropertyInfo, Type), JsonPropertyInfo> cache =
+                LazyInitializer.EnsureInitialized(ref RuntimePropertyCache, () => new ConcurrentDictionary<(JsonPropertyInfo, Type), JsonPropertyInfo>());
 
-            return runtimeProperty;
+            return cache.GetOrAdd((property, runtimePropertyType), (key, arg) => CreateRuntimeProperty(key, arg), (options, Type));
         }
     }
 }

src/System.Text.Json/src/System/Text/Json/Serialization/JsonClassInfo.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Collections;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Reflection;
@@ -27,6 +28,9 @@ internal sealed partial class JsonClassInfo
         // All of the serializable properties on a POCO (except the optional extension property) keyed on property name.
         public volatile Dictionary<string, JsonPropertyInfo> PropertyCache;
 
+        // Serializable runtime/polymorphic proerties, keyed on property and runtime type.
+        public ConcurrentDictionary<(JsonPropertyInfo, Type), JsonPropertyInfo> RuntimePropertyCache;
+
         // All of the serializable properties on a POCO including the optional extension property.
         // Used for performance during serialization instead of 'PropertyCache' above.
         public volatile JsonPropertyInfo[] PropertyCacheArray;

src/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Read.HandleArray.cs

@@ -31,7 +31,7 @@ public static partial class JsonSerializer
             }
             else if (state.Current.JsonClassInfo.ClassType == ClassType.Unknown)
             {
-                jsonPropertyInfo = state.Current.JsonClassInfo.CreatePolymorphicProperty(jsonPropertyInfo, typeof(object), options);
+                jsonPropertyInfo = state.Current.JsonClassInfo.GetOrAddPolymorphicProperty(jsonPropertyInfo, typeof(object), options);
             }
 
             // Verify that we have a valid enumerable.

src/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Read.HandleValue.cs

@@ -24,7 +24,7 @@ private static void HandleValue(JsonTokenType tokenType, JsonSerializerOptions o
             }
             else if (state.Current.JsonClassInfo.ClassType == ClassType.Unknown)
             {
-                jsonPropertyInfo = state.Current.JsonClassInfo.CreatePolymorphicProperty(jsonPropertyInfo, typeof(object), options);
+                jsonPropertyInfo = state.Current.JsonClassInfo.GetOrAddPolymorphicProperty(jsonPropertyInfo, typeof(object), options);
             }
 
             jsonPropertyInfo.Read(tokenType, ref state, ref reader);

src/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Write.Helpers.cs

@@ -31,7 +31,7 @@ private static void GetRuntimePropertyInfo(object value, JsonClassInfo jsonClass
                 // Nothing to do for typeof(object)
                 if (runtimeType != typeof(object))
                 {
-                    jsonPropertyInfo = jsonClassInfo.CreatePolymorphicProperty(jsonPropertyInfo, runtimeType, options);
+                    jsonPropertyInfo = jsonClassInfo.GetOrAddPolymorphicProperty(jsonPropertyInfo, runtimeType, options);
                 }
             }
         }

src/System.Text.Json/tests/Serialization/Object.WriteTests.cs

@@ -75,5 +75,30 @@ private class Indexer
 
             public string NonIndexerProp { get; set; }
         }
+
+        [Fact]
+        public static void WritePolymorhicSimple()
+        {
+            string json = JsonSerializer.Serialize(new { Prop = (object)new[] { 0 } });
+            Assert.Equal(@"{""Prop"":[0]}", json);
+        }
+
+        [Fact]
+        public static void WritePolymorphicDifferentAttributes()
+        {
+            string json = JsonSerializer.Serialize(new Polymorphic());
+            Assert.Equal(@"{""P1"":"""",""p_3"":""""}", json);
+        }
+
+        private class Polymorphic
+        {
+            public object P1 => "";
+
+            [JsonIgnore]
+            public object P2 => "";
+
+            [JsonPropertyName("p_3")]
+            public object P3 => "";
+        }
     }
 }